Function spawn_tipset_fetch 

fn spawn_tipset_fetch(
    network: SyncNetworkContext,
    chain_store: ChainStore,
    peer_id: Option<PeerId>,
    tipset_keys: TipsetKey,
    tipset_sender: Sender<FullTipset>,
    cancellation_token: CancellationToken,
    permit: Option<OwnedSemaphorePermit>,
)

Fetches a tipset off the event loop, forwarding a success into tipset_sender (the same channel miner tipsets use). Any permit is held for the fetch.